If I ran gvim and ran it from the desktop, I would probably need it. But I live on the command line, and I have an excellent mental model of where all the files are located. We use a reasonable directory structure for source code and support files, and it takes less time to :e and use tab completion to get to other files than it does to use NerdTree.

I think NerdTree is really nifty and a good thing, but it's just something that isn't really useful for me, given the way I work and the data I work with. But I sure do like it. I just don't use it.